[leoc]

That's what professional students of linguistics are for. Presumably when you see multiple instances of a word or family of words being rhymed in a particular way by different authors of a particular place and time, you start to get a reasonable idea of how it was originally pronounced. Sometimes people also directly comment in writing about contemporary pronunciations.

[azangru]

> That's what professional students of linguistics are for.

Professional students of linguistics will say that the terminal -y was pronounced as "eye" in the time of Shakespeare; but by the time of Blake such pronunciation has already become obsolete. However, this doesn't help us distinguish between two possibilities:

- Blake's -y rhyme is purely visual and is no more than a nod to the tradition; it is not intended to be pronounced as /ai/, or

- Poems of that time used to be delivered in an archaic pronunciation, in which case "symmetry" actually rhymed with "eye".

I don't think linguists have an answer that would convincingly point to either of the two possible pronunciations.

[leoc]

But if you're simply making a narrow point about that pronunciation then I don't see how this particularly refutes what gowld said. That there's uncertainty or ambiguity around one specific archaising use of a word in a Blake poem doesn't change the fact that we (apparently—I'm no expert) know that someone wading through stanzas of Shakespeare in a cut-glass RP accent is doing it badly wrong (at least if any of period-accuracy, rhyme or the author's intentions are important goals) and that our best guess at OP is probably a lot closer to the mark, even if it’s not certain that every single pronunciation is exactly right. (And even if it's all somewhat indeterminate since not every actor and writer in early-1600s London had exactly the same accent, etc.)